This study is aimed at defining and further defining new and known cell surface antigens of murine lymphocytes and spleen cells. Several approaches are used: 1. Production of alloantisera. Conventional inbred strains of mice are used and immunized with donor lymphocytes. Their sera are subsequently tested for antibody by cytotoxicity or other tests (see below). We are currently producing antisera to the products of the Thy-1, H-2, Ia, Ly-1, 2, 3, 4, 5, 6, 7, Lyb-2, 3, 4, 5, loci. In addition, several new specificities have been tentatively defined, which we have tentatively labelled Ly-9, 10, 11 (12, 13) - the earlier two not being clearly established. To define those new specificities, use has been made of the related strains 129 and LP.RIII. 2. Cell fusion for the production of antibodies. In addition to conventional means, we are now producing antisera by the cell fusion-hybridoma method, wherein immune spleen cells are fused with a myeloma, and high titred antibodies are subsequently produced in vitro. Using this method we have produced monoclonal antisera to H-2, Ia, Thy-1, Ly-1.1, Ly-2.1, Ly-9.2 and other specificities in the mouse. 3. Analysis of antisera: Serological methods. The distribution of the cell surface antigens has been analysed using cytotoxicity but also sensitive rosetting methods which permit the direct visualisation and isolation of cells for functional testing. In addition we have made use of the F.A.C.S. (fluorescent activated cell sorter) to analyse the distribution of all specificities - including Ly-1 and Ly-2 on T cells. 4. Functional studies. The specificities listed above are being studied in T-helper, suppressor and delayed type hypersensitivity assays and for cells responding to mitogens and in the mixed lymphocyte reaction.